The present invention relates to a semiconductor device including a anode layer having regions of low impurity density created by selective diffusion. More particularly, the present invention relates to a semiconductor device capable of removing accumulated carriers through an anode terminal coupled to an anode layer including low density regions formed by the selective diffusion of impurities. By this structure, the present invention enjoys reliable operation with low loss and with fast reverse recovery.
FIGS. 1(A), 1(B), 1(C), and 1(D) illustrate sectional views of prior art semiconductor devices. FIG. 1(A) is a sectional view of a basic vertical PN diode. FIGS. 1(B), 1(C), and 1(D) are sectional views of well-known variations of the basic PN diode structure including additional structural elements for reducing reverse recovery time.
Conventional semiconductor devices generally suffer from high electrical loss and/or slow reverse recovery time. In conventional devices, electrical charge in the form of excess minority carders accumulates in forward biased diodes. This charge switches from a forward biased direction to a reversed biased direction in proportion to a externally switched forward biased current. During this switching period, a diode having a high breakdown voltage requires a high internal resistance which results in unacceptably long reverse recovery time (i.e., recombination time). This adverse affect and the resulting electrical loss increases with the switching frequency of the diode.
Accordingly, semiconductor devices operating at high switching frequencies typically become more complex in an attempt to compensate for the adverse affect described above. Exemplary of these more complex circuits is the Schottky barrier diode. However, the Schottky barrier diode suffers from a high leakage current and low breakdown voltage. To avoid these disadvantages, the prior art often incorporates a lifetime killer dopant within the basic diode structure shown in FIG. 1(A) in order to reduce reverse time recovery.
Referring to FIG. 1(A), the conventional diode structure includes a cathode electrode 2 in ohmic contact with a N.sup.+ semiconductor layer 3. A N.sup.- semiconductor layer 4 is formed over N.sup.+ layer 3. As customarily indicated by the (+ and -), N.sup.- layer 4 has an impurity concentration, or density, relatively lower than N.sup.+ layer 3. With respect to semiconductor layer 3 and 4, as well as all subsequent semiconductor layers described below, a determination of the exact impurity densities is the subject to various design considerations and is widely considered to be within the range of ordinary skill in the art. Only type and the relative density of the impurities are of importance with respect to the present invention described below.
Referring again to FIG. 1(A), a P.sup.+ well region 6 is formed by diffusing P-type impurities into a major surface of N.sup.- layer 4. Finally, an anode electrode 1 is overlaid upon the major surface in ohmic contact with P.sup.+ well region 6 through a insulating mask layer 5. The above-mentioned electrical loss for the conventional semiconductor diode structure shown in FIG. 1(A) results from a high voltage drop and a high leakage current at high temperature.
In an attempt to reduce the reverse recovery time associated with the basic diode structure above, the prior art has proposed the structure shown in FIG. 1(B). This structure differs from the one shown in FIG. 1(A) in that the P.sup.+ well region 6 is replaced by a complex structure including a plurality of deep P.sup.+ well regions 6 joined by intervening shallow P.sup.- regions 7. The plurality of P.sup.+ well regions 6 are generally formed by impurity, such as boron, diffusion. On the other hand, P.sup.- regions 7 are formed by ion implantation.
The structures shown in FIGS. 1(C) and 1(D) are additional prior attempts to reduce reverse recovery time and voltage drop. The structure shown in FIG. 1(C) includes a P-type layer formed over N.sup.- layer 4 and a well structure formed in a major surface of the P-type layer including alternating P.sup.+ regions 6 and N.sup.+ regions 8. The structure shown in FIG. 1(D) replaces the shallow P.sup.- regions 7 of the structure in FIG. 1(B) with Schottky barrier contacts 9.
The resulting structures in FIGS. 1(B), 1(C), and 1(D) enjoy better reverse recovery time and lower voltage drop than the basic diode structure of FIG. 1(A). However, these advantages come at the price of structural complexity and the associated additional manufacturing steps.